[CRICKETS] [FEET PATTERING] [SWISH] [BING] [SPLASH] [MUSIC PLAYING] [CROAKING] SEAN B. CARROLL: Our planet
has millions of species-- over 300,000 beetles
alone, 17,000 butterflies-- thousands of mammals,
fish, and birds, all astonishingly different. How did so many
species come to be? To seek insights
into that question, researchers are
focusing on places where species recently arose,
such as the remote Galapagos Islands. Scientists are
making observations and conducting
experiments that would have surprised Charles Darwin. And they're discovering
new insights into what the great
naturalist called the mystery of mysteries-- how new species form. [MUSIC PLAYING] [BIRDS CAWING] The Galapagos Islands are one of
the most spectacular landscapes in the world, home to
a variety of species that live nowhere else. Biologists Peter
and Rosemary Grant have been seeking
answers to how species arise by focusing on one
of the smaller islands called Daphne Major. PETER GRANT: When
we started out, we had no plan
for the long-term. In fact, we thought
it was just going to be just a few
years, maybe two years. SEAN B. CARROLL: Two years have
turned into a 40 year odyssey. The Grants have returned
every summer since 1973. ROSEMARY GRANT:
Oh, there's a bird. PETER GRANT: Is that 306? ROSEMARY GRANT: OK, that's-- PETER GRANT: 306. ROSEMARY GRANT: 3-0, Metal 6. SEAN B. CARROLL:
Here they've made some of the most
remarkable observations in the history of field
research as they studied the famed Galapagos finches. [BIRDS CAWING] [MUSIC PLAYING] The finches were first brought
to scientists' attention by Charles Darwin, when his
voyage around South America brought him to this
cluster of islands 600 miles from mainland Ecuador. These volcanic islands
are geologically young. They began rising
from the ocean floor less than 5 million years ago. At first devoid
of life, they now support a modest
number of species. Among them, 13
species of finches found in various combinations
on the different islands. The birds live in
diverse habitats. ROSEMARY GRANT: The islands are
very different from each other. They differ in size. They differ in
topography and in height. SEAN B. CARROLL: Larger trees
grow at higher elevations, while low islands have mostly
cactus, grasses, and shrubs. In these diverse
habitats, the finches have evolved many
ways to survive. So, Rosemary, what's
the important difference between these birds? ROSEMARY GRANT: This
little warbler finch, with its very fine,
needle-like beak, is perfect for
picking off insects. This one is the woodpecker finch
with a rather more robust beak. It concentrates on beetle
larvae and termite larvae. Then we have a cactus finch
with a much longer sharp-pointed beak, which probes
into cactus flowers. And then these three species
are the large, medium, and small ground finches. So, Sean, the basic idea
is the beaks are tools, and you need the right
tool for the right job. [BIRDS CHIRPING] [MUSIC PLAYING] SEAN B. CARROLL:
The finches look so different that Darwin
first mistook them for entirely unrelated
kinds of birds. How did the Galapagos end
up with so many species of finches? In terms of the actual
history of the finches of the Galapagos, there were
many different possibilities. Different times,
those finches could have all come from the
mainland separately or the finches could have
all evolved out there on the islands. And what do we know about that? PETER GRANT: Well, now
we know from DNA evidence that all of the finches are
more related to each other than any one is to a
species on the mainland. And that tells us
only one species arrived on the Archipelago and
diversified into the 13 species that we see nowadays
in the Galapagos. So they've all come from
a single common ancestor. SEAN B. CARROLL:
The question then becomes, how did one
ancestral population give rise to many different
species, each adapted to a different lifestyle? A crucial insight into
how adaptation occurs came when the Grants
focused on one species on the island of Daphne Major. PETER GRANT: A factor a
great convenience for us was the small size
of the island. [BIRDS CLUCKING] That meant that we could
walk all over the place. ROSEMARY GRANT:
Oh, there's a bird. PETER GRANT: I'll
leave that one to you. ROSEMARY GRANT: The idea was
that if we worked really hard, we could follow every individual
or almost every individual. SEAN B. CARROLL: They
rose at 5:30 each morning to net the island's
medium ground finches. They measured the size and shape
of each bird's beak, the birds weight, and they tagged
them for identification. PETER GRANT: The
male is in 17418. SEAN B. CARROLL:
Year after year, they returned, at times tracking
over 1,000 finches. PETER GRANT: So
here's an example of a bird we know intimately
over the whole of its lifespan. The number is 5960. We know how many times it
bred, which years it bred in, how many mates it had, how
many offspring it produced, and then how many
of those offspring themselves survive
long enough to breed. [MUSIC PLAYING] SEAN B. CARROLL: Over
the first four years, little seemed to change. Then in 1977, a
terrible drought began. PETER GRANT: Virtually no rain
fell for the next 18 months. ROSEMARY GRANT: The
vegetation practically disappeared, apart
from a few trees without any leaves
and, of course, the cactus bushes
were still there. SEAN B. CARROLL: Now the
medium ground finches had to compete for scarce food. PETER GRANT: They started
off with a big food supply of small seeds,
medium seeds, large seeds. As these small seeds
became very scarce, they had to turn increasingly
to the large and hard seeds. But only birds with large
beaks can crack open these woody, spiny fruits. SEAN B. CARROLL: The birds
with the smallest beaks had the most trouble. ROSEMARY GRANT:
They were scraping about amongst the rocks, and
their plumage got so worn that they could barely fly. SEAN B. CARROLL:
That year, over 80% of the medium
ground finches died. PETER GRANT: We would
go around looking for birds that had died,
and it's very sad to pick up a bird and say, "3972-- oh, no. Not that bird. Oh!" SEAN B. CARROLL:
When they inventoried the surviving medium
ground finches, they discovered
that one trait had made the greatest difference
between life and death. PETER GRANT: What
I'm showing here, a distribution of beak depths
of the population in 1976. The survivors of this
group are shown in black. SEAN B. CARROLL: So
the larger the beak, the better your chances. PETER GRANT: The
larger the beak, the higher the
likelihood of surviving through the drought of 1977. [MUSIC PLAYING] 18.6 grams. SEAN B. CARROLL: When they
looked at the offspring, they found an even
greater surprise. The average beak
depth was more than 4% larger than the
previous generation. Natural selection had changed
the average beak size. Could you have ever imagined
measuring and observing something like this on
such a short timescale, until you actually did it? PETER GRANT: When we
started, the answer is no. We could not imagine we
would be able to do it. [MUSIC PLAYING] SEAN B. CARROLL: But was this a
fluke or are changes like this happening all the time? [THUNDER RUMBLING] Five years later, in 1983,
an unusually strong El Nino brought 10 times more
rain than normal, and the island was
overrun by vines that covered even the cactus. The rains changed the vegetation
on the island, such that two years later, when
drought struck, larger seeds became scarce. The birds with larger beaks
now had difficulty picking up the more abundant
food, the small seeds produced by the vines. That year, many more finches
with small beaks survived. And their offspring
inherited smaller beaks. PETER GRANT: So
the selection had swung in the opposite
direction, and evolution had occurred as a result. SEAN B. CARROLL: In an
amazingly short period of time, the Grants had measured
evolution of beak size not once, but
twice, demonstrating that when birds encountered
different environments they will change over a
very short amount of time. [MUSIC PLAYING] Over millions of years,
changes like these occurring throughout the Galapagos
generated all sorts of beak sizes and shapes. But that's only
part of the story. How did finches with different
beaks become distinct species? Species are defined
as populations whose members don't interbreed. So how does one
species split into two? A typical scenario is that
two populations become separated geographically
and undergo enough change in their respective
habitats that, if, or when, they come into contact
again, they do not mate. [MUSIC PLAYING] So in the Galapagos,
the Grants asked, what keeps different species
of finches from mating? [BIRDS CHIRPING] ROSEMARY GRANT: We
were very conscious that, on any given island,
the different species sang very different songs. This is what a cactus
finch sounds like-- [CACTUS FINCH CHIRPING] --whereas the medium ground
finch sounds very much like this-- [MEDIUM GROUND FINCH CHIRPING] [MUSIC PLAYING] SEAN B. CARROLL: So to see if
songs keep the species apart, the Grants and their
student, Laureen Ratcliffe, played each species' songs
through a loudspeaker. [BIRDS CHIRPING] ROSEMARY GRANT: When we played
back the cactus finch song, cactus finch came
to the loudspeaker and the medium ground finch
completely ignored it. SEAN B. CARROLL: The
males only responded to songs of their own species. The Grants looked at whether
finches might also choose mates based on appearance. So they put out stuffed
female specimens to see if males would respond. ROSEMARY GRANT: Clearly,
they could discriminate. The male vigorously courted
a female of his own species. Completely ignored
the other one. SEAN B. CARROLL: The
males only courted females that had a similar
size and similar beak. Song and appearance
both play a role in keeping different
species from mating. [MUSIC PLAYING] So when populations of the
same species are separated, changes in these
traits set the stage for the formation
of new species. The Grants have shown that
both geography and ecology are keys to the evolution
of the Galapagos finches. The most likely scenario is
that two million years ago a single finch population
arrived from the mainland. When their descendants
reached another island, they faced new conditions. As those isolated populations
adapted to their surroundings, their traits changed. If the changes included
traits involved in mating, and the populations
came into contact again, they no longer mated. They had become
distinct species. While unique to
these remote islands, the history of the
Galapagos finches offers a general insight
into why the world is populated with so many species. PETER GRANT: The more
diverse the environment, the more opportunities
for evolutionary change to produce those new species. [BIRDS CAWING] SEAN B. CARROLL: Over 150
years after Darwin first recognized their significance,
these unassuming birds still illuminate how the
great diversity of life arose and continues to evolve. [MUSIC PLAYING] [MUSIC PLAYING]
